If either of the contributing threads should break upstream of the merger point, the remaining thread would continue for some time to be drawn by the tensioning means to produce a defective length of yarn even if the machine were promptly deactivated--manually or automatically--in response to a signal from a sensor detecting the break. It is therefore desirable that a thread guard have means responsive to a rupture of either thread for quickly breaking the other thread in order to prevent the wasting of a significant portion of that thread and to avoid the production of a defective yarn section. Thus, German utility model No. GM 79 12 423 discloses a thread guard of this nature comprising a swingable member disposed just downstream of a thread junction for guiding engagement with the yarn in a normal position of that member from which it is limitedly displaceable to one side or the other by minor differences in the tension of the contributing threads. When this difference exceeds a certain threshold, as will be the case in the event of a thread rupture, the guide member is deflected into an off-normal position in which it impedes the advance of the intact thread so as to cause its rupture. In a specific instance, the guide member is balanced on a horizontal pin and its normal position is metastable so that gravity makes it rotate through 180.degree. when the limits of lateral deflection are surpassed whereby two pins bracketing the yarn invert their relative position to entangle the remaining thread. Also mentioned is the possibility that gravity be replaced by some other stored force, such as the stress of a spring or an electromagnetic field, to create something like a toggle effect when the guided yarn strongly deviates from its regular path. In any event, the yarn is guided with considerable lateral play by the swingable member in its metastable normal position.